U.S. patent application number 15/685726 was filed with the patent office on 2018-01-04 for material for electrode of power storage device, power storage device, and electrical appliance.
The applicant listed for this patent is SEMICONDUCTOR ENERGY LABORATORY CO., LTD.. Invention is credited to Nobuhiro INOUE, Kai KIMURA, Kazutaka KURIKI, Junpei MOMO, Tamae MORIWAKA, Teppei OGUNI, Ryota TAJIMA, Shunpei YAMAZAKI.
Application Number | 20180005761 15/685726 |
Document ID | / |
Family ID | 50408178 |
Filed Date | 2018-01-04 |
United States Patent
Application |
20180005761 |
Kind Code |
A1 |
INOUE; Nobuhiro ; et
al. |
January 4, 2018 |
MATERIAL FOR ELECTRODE OF POWER STORAGE DEVICE, POWER STORAGE
DEVICE, AND ELECTRICAL APPLIANCE
Abstract
To improve the reliability of a power storage device. A granular
active material including carbon is used, and a net-like structure
is formed on part of a surface of the granular active material. In
the net-like structure, a carbon atom included in the granular
active material is bonded to a silicon atom or a metal atom through
an oxygen atom. Formation of the net-like structure suppresses
reductive decomposition of an electrolyte solution, leading to a
reduction in irreversible capacity. A power storage device using
the above active material has high cycle performance and high
reliability.
Inventors: |
INOUE; Nobuhiro; (Atsugi,
JP) ; TAJIMA; Ryota; (Isehara, JP) ; MORIWAKA;
Tamae; (Isehara, JP) ; MOMO; Junpei;
(Sagamihara, JP) ; OGUNI; Teppei; (Atsugi, JP)
; KIMURA; Kai; (Atsugi, JP) ; KURIKI;
Kazutaka; (Ebina, JP) ; YAMAZAKI; Shunpei;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SEMICONDUCTOR ENERGY LABORATORY CO., LTD. |
Atsugi-shi |
|
JP |
|
|
Family ID: |
50408178 |
Appl. No.: |
15/685726 |
Filed: |
August 24, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14032214 |
Sep 20, 2013 |
9754728 |
|
|
15685726 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 10/0525 20130101;
Y02E 60/10 20130101; H01G 11/06 20130101; H01M 4/134 20130101; H01G
11/50 20130101; H01G 11/26 20130101; H01M 4/366 20130101; H01M
4/133 20130101; H01M 4/02 20130101; Y02E 60/13 20130101; H01G 9/042
20130101 |
International
Class: |
H01G 9/042 20060101
H01G009/042; H01G 11/26 20130101 H01G011/26; H01G 11/50 20130101
H01G011/50; H01G 11/06 20130101 H01G011/06; H01M 4/133 20100101
H01M004/133; H01M 4/02 20060101 H01M004/02; H01M 4/134 20100101
H01M004/134; H01M 4/36 20060101 H01M004/36 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 9, 2012 |
JP |
2012-224581 |
Claims
1. (canceled)
2. A material for an electrode of a power storage device
comprising: a granular active material comprising a carbon atom;
and a film having a structure over the granular active material,
wherein the structure comprises a plurality of bonds between the
carbon atom and one of a silicon atom and a metal atom through an
oxygen atom, and wherein the film partly covers a surface of the
granular active material so that the surface of the granular active
material has a first region which is covered with the film, and a
second region which is not covered with the film.
3. The material for an electrode of a power storage device
according to claim 2, wherein the granular active material
comprises a graphite particle.
4. The material for an electrode of a power storage device
according to claim 2, wherein the metal atom is one of a niobium
atom, a titanium atom, a vanadium atom, a tantalum atom, a tungsten
atom, a zirconium atom, a molybdenum atom, a hafnium atom, a
chromium atom, and an aluminum atom.
5. The material for an electrode of a power storage device
according to claim 2, further comprising a plurality of oxide
layers over the film, wherein each of the plurality of oxide layers
comprises a bond of the one of the silicon atom and the metal atom,
and an oxygen atom.
6. A power storage device comprising a negative electrode, the
negative electrode comprising the material for an electrode of a
power storage device according to claim 2.
7. An electrical appliance comprising the power storage device
according to claim 6.
8. A material for an electrode of a power storage device
comprising: a granular active material comprising a carbon atom;
and a film having a structure over the granular active material,
wherein the structure comprises a plurality of bonds between the
carbon atom and a silicon atom through an oxygen atom, and wherein
the film partly covers a surface of the granular active material so
that the surface of the granular active material has a first region
which is covered with the film, and a second region which is not
covered with the film.
9. The material for an electrode of a power storage device
according to claim 8, wherein the granular active material
comprises a graphite particle.
10. The material for an electrode of a power storage device
according to claim 8, further comprising a plurality of oxide
layers over the film, wherein each of the plurality of oxide layers
comprises a bond of the silicon atom and an oxygen atom.
11. A power storage device comprising a negative electrode, the
negative electrode comprising the material for an electrode of a
power storage device according to claim 8.
12. An electrical appliance comprising the power storage device
according to claim 11.
13. A material for an electrode of a power storage device
comprising: a granular active material comprising a carbon atom;
and a film having a structure over the granular active material,
wherein the structure comprises a plurality of bonds between the
carbon atom and one of a silicon atom and a metal atom through an
oxygen atom, wherein the granular active material comprises a
plurality of graphene layers, wherein the film partly covers a
surface of the granular active material so that the surface of the
granular active material has a first region which is covered with
the film, and a second region which is not covered with the film,
and wherein the structure exists at end portions of part of the
plurality of graphene layers.
14. The material for an electrode of a power storage device
according to claim 13, wherein the metal atom is one of a niobium
atom, a titanium atom, a vanadium atom, a tantalum atom, a tungsten
atom, a zirconium atom, a molybdenum atom, a hafnium atom, a
chromium atom, and an aluminum atom.
15. The material for an electrode of a power storage device
according to claim 13, further comprising a plurality of oxide
layers over the film, wherein each of the plurality of oxide layers
comprises a bond of the one of the silicon atom and the metal atom,
and an oxygen atom.
16. A power storage device comprising a negative electrode, the
negative electrode comprising the material for an electrode of a
power storage device according to claim 13.
17. An electrical appliance comprising the power storage device
according to claim 16.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a material for an electrode
of a power storage device, a power storage device, and an
electrical appliance.
2. Description of the Related Art
[0002] In recent years, a variety of power storage devices, for
example, non-aqueous secondary batteries such as lithium ion
batteries (LIBs), lithium ion capacitors (LICs), and air cells have
been actively developed. In particular, demand for lithium ion
batteries with high output and high energy density has rapidly
grown with the development of the semiconductor industry, for
example, portable information terminals such as mobile phones,
smartphones, and laptop computers, portable music players, and
digital cameras; medical equipment; and next-generation clean
energy vehicles such as hybrid electric vehicles (REVS), electric
vehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). The
lithium ion batteries are essential for today's information society
as rechargeable energy supply sources.
[0003] A negative electrode of the power storage devices such as
the lithium ion batteries and the lithium ion capacitors includes
at least a negative electrode current collector and a negative
electrode active material layer provided on a surface of the
negative electrode current collector. The negative electrode active
material layer contains a negative electrode active material such
as carbon or silicon, which can store and release lithium ions
serving as carrier ions.
[0004] A negative electrode of a lithium ion battery using a
graphite-based carbon material is formed by mixing graphite (black
lead) that is a negative electrode active material, acetylene black
(AB) as a conductive additive, and polyvinylidene fluoride (PVDF)
that is a resin as a binder to form slurry, applying the slurry
over a negative electrode current collector, and drying the slurry,
for example.
[0005] A lithium ion battery or a lithium ion capacitor has a
problem in that irreversible capacity caused by repetitive
insertion/extraction of lithium ions into/from the negative
electrode active material is generated.
[0006] A negative electrode of a lithium ion battery or a lithium
ion capacitor has an extremely low electrode potential and a high
reducing ability. Accordingly, an electrolyte solution containing
an organic solvent is reduced and decomposed, and decomposed
matters form a film on a surface of the negative electrode. The
formation of the film generates irreversible capacity, so that part
of discharge capacity is lost.
[0007] As a technique for reducing the loss of discharge capacity,
for example, a technique in which a surface of a negative electrode
active material is covered with a metal oxide film, a silicon oxide
film, or the like has been known (e.g., Patent Document 1). The
formation of the above oxide film on the surface of the negative
electrode active material can suppress formation of the film formed
on the surface of the negative electrode due to the decomposition,
and thus can reduce irreversible capacity.
REFERENCE
Patent Document
[0008] [Patent Document 1] Japanese Published Patent Application
No. H11-329435
SUMMARY OF THE INVENTION
[0009] However, capacity loss cannot be reduced sufficiently in
conventional power storage devices.
[0010] For example, in the case of using a carbon-based material as
a negative electrode active material, the cause of generation of
irreversible capacity is not only the film formed on the surface of
the negative electrode due to the decomposition, but can be, for
example, a functional group or a dangling bond at an end of the
negative electrode active material. The functional group or the
dangling bond is unstable and a structure of the carbon-based
material is readily changed, so that irreversible capacity is
likely to be formed.
[0011] A functional group or a dangling bond exists even when a
surface of a negative electrode active material is covered with an
oxide film as in, for example, Patent Document 1. Therefore, the
capacity loss cannot be reduced sufficiently only by the
conventional method of covering a surface of a negative electrode
active material with an oxide film.
[0012] The above problems exist not only in lithium ion batteries
but also in lithium ion capacitors.
[0013] An object of one embodiment of the present invention is to
reduce irreversible capacity of a power storage device.
[0014] An object of one embodiment of the present invention is to
reduce the number of functional groups or dangling bonds at an end
of a material serving as an active material.
[0015] Another object of one embodiment of the present invention is
to improve the reliability of a power storage device.
[0016] In one embodiment of the present invention, a granular
active material including carbon is used, and a net-like structure
is formed on part of a surface of the granular active material. The
net-like structure is formed by a plurality of bonds between a
carbon atom included in the granular active material and a silicon
atom or a metal atom through an oxygen atom. Formation of the
net-like structure suppresses reductive decomposition of an
electrolyte solution, leading to a reduction in irreversible
capacity. Furthermore, the number of functional groups or dangling
bonds existing on the surface of the granular active material is
reduced in order to reduce irreversible capacity.
[0017] One embodiment of the present invention is a material for an
electrode of a power storage device, which includes a granular
active material and has a net-like structure on part of a surface
of the granular active material. The net-like structure is formed
by a plurality of bonds between a carbon atom included in the
granular active material and a silicon atom or a metal atom through
an oxygen atom.
[0018] In the above embodiment of the present invention, for
example, in the case where the granular active material is a
graphite particle including a plurality of graphene layers, a
net-like structure may be provided across ends of the plurality of
graphene layers on part of the surface of the granular active
material. Thus, a change in structure, such as separation of
graphene layers by insertion/extraction of carrier ions into/from
graphite particles, is suppressed.
[0019] In the above embodiment of the present invention, n (n is a
natural number) oxide layers each including a bond of the silicon
atom or the metal atom and the oxygen atom may be provided over the
net-like structure.
[0020] Another embodiment of the present invention is a power
storage device in which a negative electrode active material layer
of a negative electrode includes the above material for an
electrode of a power storage device.
[0021] Another embodiment of the present invention is an electrical
appliance including the above power storage device.
[0022] In one embodiment of the present invention, irreversible
capacity can be reduced, and thus, loss of discharge capacity can
be reduced. Furthermore, the number of functional groups or
dangling bonds at an end of a material serving as an active
material can be reduced. Moreover, the reliability of a power
storage device can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIGS. 1A and 1B illustrate an example of a material for an
electrode of a power storage device.
[0024] FIGS. 2A and 2B illustrate an example of a material for an
electrode of a power storage device.
[0025] FIG. 3 illustrates an example of a material for an electrode
of a power storage device.
[0026] FIGS. 4A and 4B are each a flow chart of a method for
producing a material for an electrode of a power storage
device.
[0027] FIGS. 5A to 5D illustrate an example of a negative electrode
of a power storage device.
[0028] FIGS. 6A to 6C illustrate an example of a negative electrode
of a power storage device.
[0029] FIGS. 7A and 7B illustrate an example of a power storage
device.
[0030] FIG. 8 illustrates an example of a power storage device.
[0031] FIGS. 9A and 9B illustrate an example of a power storage
device.
[0032] FIG. 10 illustrates examples of electrical appliances.
[0033] FIGS. 11A to 11C illustrate examples of electrical
appliances.
[0034] FIGS. 12A and 12B illustrate an example of an electrical
appliance.
[0035] FIG. 13 is a graph showing results of CV measurement.
[0036] FIG. 14 is an image observed with SEM.
[0037] FIGS. 15A and 15B show images observed with STEM and results
of EDX.
[0038] FIGS. 16A and 16B are graphs showing results of CV
measurement.
[0039] FIG. 17A is a graph showing results of CV measurement, and
FIG. 17B is a graph showing the capacity of decomposition of an
electrolyte solution.
[0040] FIG. 18 is a graph showing cycle performance.
DETAILED DESCRIPTION OF THE INVENTION
[0041] Embodiments of the present invention will be described
below. Note that it will be readily appreciated by those skilled in
the art that contents of the embodiments can be modified without
departing from the spirit and scope of the present invention. Thus,
the present invention should not be limited to, for example, the
description of the following embodiments.
[0042] Note that the contents in different embodiments can be
combined with one another as appropriate. In addition, the contents
in different embodiments can be replaced with one another as
appropriate.
[0043] The ordinal numbers such as "first" and "second" are used to
avoid confusion between components and do not limit the number of
each component.
Embodiment 1
[0044] In this embodiment, an example of a material for an
electrode of a power storage device will be described.
<Structural Example of Electrode Material of Power Storage
Device>
[0045] First, a structural example of a material for an electrode
of a power storage device of this embodiment will be described with
reference to FIGS. 1A and 1B, FIGS. 2A and 2B, and FIG. 3.
[0046] The material for an electrode of a power storage device
includes a granular active material 110 as illustrated in FIG.
1A.
[0047] The shape of the granular active material 110 is not
particularly limited. The granular active material 110 may have a
spherical shape (powdered state), a plate-like shape, an angular
shape, a column shape, a needle-like shape, or a scale-like shape,
for example. Note that a film-like active material may be used
instead of the granular active material 110.
[0048] As a material of the granular active material 110, a
carbon-based material (e.g., graphite) can be used.
[0049] Graphite is a layered compound in which a plurality of
graphene layers is stacked in parallel to each other by van der
Waals forces. The graphene layer is a sheet composed of a hexagonal
net pattern of a one-atom thick layer of carbon formed by carbon
atoms which are covalently bonded to each other to form sp.sup.2
hybrid orbitals and tricoordinate with each other at an angle of
120.degree. in a surface. Note that the graphene layer may partly
include defects or functional groups.
[0050] Examples of graphite include low crystalline carbon such as
soft carbon and hard carbon and high crystalline carbon such as
natural graphite, kish graphite, pyrolytic graphite, mesophase
pitch based carbon fiber, meso-carbon microbeads, mesophase
pitches, petroleum-based or coal-based coke, and the like.
[0051] The particle diameter of the granular active material 110 is
not particularly limited, and may be greater than or equal to 6
.mu.m and less than or equal to 30 .mu.m, for example.
[0052] A net-like structure of C--O-M bonds illustrated in FIG. 1B
is formed on part of a surface of the granular active material 110.
The C--O-M bond is a bond where a plurality of carbon atoms
included in the granular active material 110 is bonded to a silicon
atom or a metal atom through an oxygen atom. C represents a carbon
atom included in the granular active material 110, O represents an
oxygen atom, and M represents, for example, a niobium atom, a
titanium atom, a vanadium atom, a tantalum atom, a tungsten atom, a
zirconium atom, a molybdenum atom, a hafnium atom, a chromium atom,
an aluminum atom, or a silicon atom.
[0053] It is preferable that the net-like structure do not have
electron conductivity. Furthermore, it is preferable that the
net-like structure have a function of allowing passage of a carrier
ion of a power storage device. Note that examples of the carrier
ion include a lithium ion that is used for a lithium ion battery or
a lithium ion capacitor; an alkali metal ion (e.g., a sodium ion or
a potassium ion); an alkaline earth metal ion (e.g., a calcium ion,
a strontium ion, or a barium ion, a beryllium ion, or a magnesium
ion).
[0054] The net-like structure is preferably formed not on the
entire surface of the granular active material 110 but on part of
the surface of the granular active material 110. In the case where
a plurality of granular active materials 110 is in contact with
each other as illustrated in FIG. 1A, the net-like structure is
preferably formed in a region other than the contact portions. The
net-like structure covers not the entire surface of the granular
active material 110, so that cell reaction is possible and the
decomposition reaction of an electrolyte solution can be
suppressed.
[0055] The net-like structure illustrated in FIG. 1B is referred to
as a chemical network structured interface (CNSI) layer.
[0056] The CNSI layer is a three-dimensional net-like structure
that is formed by chemical bonds of carbon included in the granular
active material and an oxide of metal, silicon, or the like.
[0057] The CNSI layer can make a surface of the granular active
material stable as compared with a film formed on an electrode
surface due to decomposition of an electrolyte solution. Thus, the
CNSI layer functions as a protection layer. The CNSI layer is dense
and has good adhesion to the granular active material. For this
reason, providing the CNSI layer can reduce an area of the granular
active material in direct contact with an electrolyte solution,
suppress the decomposition of the electrolyte solution in a power
storage device, and reduce irreversible capacity that causes a
decrease in the initial capacity of the power storage device.
[0058] Note that n (n is a natural number) oxide layers each
including a M-O bond formed of M and O may be provided over the
net-like structure. In such a case, O in the oxide layer that is
adjacent to the CNSI layer bonds to M in the CNSI layer. Since the
same oxide is used for the CNSI layer and the oxide layer, a
connection between the CNSI layer and the oxide layer is
stable.
[0059] The netlike structure can be formed, for example, in such a
manner that a coating film 111 (a film formed of an oxide including
O and M) is formed on part of the surface of the granular active
material 110 as illustrated in FIG. 1A, and carbon included in the
granular active material 110 is bonded to an oxide included in the
coating film 111. In that case, an oxygen atom included in the
coating film 111 is represented by O, and a metal atom or a silicon
atom included in the coating film 111 is represented by M.
[0060] The coating film of one embodiment of the present invention
is an artificial film provided in advance before a power storage
device is charged and discharged, and is clearly distinguished from
a film formed due to the decomposition reaction between an
electrolytic solution and an active material in this specification
and the like.
[0061] A structural example of the material for an electrode of a
power storage device, which includes the CNSI layer, will be
described with reference to FIGS. 2A and 2B. Here, description is
made on a case of forming the netlike structure in such a manner
that a coating film (a film formed of an oxide including a metal
atom or a silicon atom) is formed on graphite particles, and carbon
included in the graphite particles is bonded to an oxide included
in the coating film. FIG. 2A is a schematic view of graphite
particles used for the netlike structure. FIG. 2B is a schematic
view illustrating the case of forming the netlike structure using
the graphite particles in FIG. 2A. Note that in FIGS. 2A and 2B, a
relatively small black sphere represents a carbon (C) atom, a
relatively large black sphere represents a metal or silicon (M)
atom, a gray sphere represents an oxygen (O) atom, and a white
sphere represents a hydrogen (H) atom. Note that for convenience,
the size of the spheres may be different from the actual size of
the atoms.
[0062] As illustrated in FIG. 2A, a graphite particle 211 is
composed of a plurality of graphene layers, and a dangling bond or
a functional group such as an OH group or a COOH group exists at an
end of the graphene layer. At this time, an end of the graphite
particle 211 is unstable. Accordingly, when the graphite particle
211 is used for a power storage device, decomposition of an
electrolyte solution and separation of graphene layers due to
insertion and extraction of carrier ions are likely to occur.
[0063] When a coating film (a film of an oxide) is formed over the
graphite particle 211 illustrated in FIG. 2A and carbon included in
the graphite particle is bonded to an oxide included in the coating
film, dangling bonds or functional groups at ends of the plurality
of graphene layers react with oxides as illustrated in FIG. 2B.
[0064] At this time, bond portions of oxides and carbon atoms at
the ends of the plurality of graphene layers on part of a surface
of the graphite particle 211 (on part of the surface of the
granular active material 110) correspond to a net-like structure
212 formed of C--O-M bonds. In other words, the net-like structure
212 is three-dimensionally formed across ends of the plurality of
graphene layers on the surface of the granular active material 110.
With the net-like structure 212 formed across ends of the plurality
of graphene layers, separations of graphene layers due to insertion
and extraction of carrier ions can be prevented; therefore, a
change in the structure of the graphite particle 211 can be
prevented.
[0065] Moreover, n oxide layers 213 are formed over the net-like
structure 212. Note that the oxide layer 213 may be extended to the
surface of the graphite particle 211 positioned in a portion other
than the ends of the plurality of graphene layers.
[0066] FIG. 3 is a schematic diagram illustrating the case where
the electrode material illustrated in FIG. 2B is in contact with an
electrolyte solution. As illustrated in FIG. 3, when the
three-dimensional net-like structure 212 formed of C--O-M bonds and
the oxide layer 213 are provided between the graphite particle 211
and an electrolyte solution 214 including a molecule containing
lithium, an area of the graphite particle 211 in direct contact
with the electrolyte solution 214 is reduced, so that decomposition
of the electrolyte solution 214 is suppressed. Moreover, a film due
to the decomposition of the electrolyte solution is less likely to
be formed on the surface of the graphite particle 211 including the
net-like structure 212.
[0067] As described above with reference to FIGS. 1A and 1B, FIGS.
2A and 2B, and FIG. 3, in the material for an electrode of a power
storage of this embodiment, the net-like structure formed of C--O-M
bonds is provided on part of the surface of the granular active
material, so that the surface of the granular active material is
stabilized. Thus, decomposition of the electrolyte solution can be
suppressed.
<Method of Producing Electrode Material of Power Storage
Device>
[0068] Next, as an example of a method for producing the material
for an electrode of a power storage device that includes the
netlike structure, a method for producing the material for an
electrode of a power storage device illustrated in FIG. 1A will be
described with reference to FIGS. 4A and 4B. Here, a producing
method using a sol-gel method and a producing method using a
polysilazane method will be described as examples.
[Method of Producing Electrode Material of Power Storage Device
Using Sol-Gel Method]
[0069] At Step S150 in FIG. 4A, metal alkoxide or silicon alkoxide
and a stabilizing agent are added to a solvent, and the mixture is
stirred to form a solution.
[0070] As the solvent, toluene can be used, for example.
[0071] As the stabilizing agent, ethyl acetoacetate can be used,
for example.
[0072] When a silicon oxide film is formed as the coating film 111,
for example, silicon ethoxide, methoxide, or the like can be used
as alkoxide.
[0073] Next, at Step S151, the granular active material 110 is
added to the solution, and the mixture is stirred. Thus, the
solution becomes a thick paste and the surface of the granular
active material 110 is covered with the alkoxide.
[0074] At Step S152, the alkoxide on the surface of the granular
active material 110 is turned into a gel by a sol-gel method.
[0075] At Step S152, a small amount of water is added to the
solution including the granular active material 110 so that the
alkoxide reacts with water, whereby a sol-state decomposition
product is formed. Here, the term "a sol state" refers to a state
where solid fine particles are substantially uniformly dispersed in
a liquid. Note that the small amount of water may be added to the
solution including the active material by exposing the solution to
the air. For example, in the case where silicon ethoxide
(Si(OEt).sub.4) is used as the alkoxide, hydrolysis reaction is
expressed by Formula 1.
Si(OEt).sub.4+4H.sub.2O.fwdarw.Si(OEt).sub.4-x(OH).sub.x+xEtOH (x
is an integer of 4 or less) (Formula 1)
[0076] Next, the sol-state decomposition product is dehydrated and
condensed to be a reactant which is a gel. Here, "being a gel"
refers to being in a state where a three-dimensional network
structure is developed due to attractive interaction between solid
fine particles and the decomposition product is solidified. In the
case where silicon ethoxide (Si(OEt).sub.4) is used as the
alkoxide, the condensation reaction is expressed by Formula 2.
2Si(OEt).sub.4-x(OH).sub.x.fwdarw.(OEt).sub.4-x(OH).sub.x-1Si--O--Si(OH)-
.sub.x-1(OEt).sub.4-x+H.sub.2O (x is an integer of
4 or less) (Formula 2)
[0077] When silicon ethoxide is used as the alkoxide and a graphite
particle is used as the granular active material 110, condensation
reaction of hydrate of silicon ethoxide occurs, whereby the
net-like structure formed of C--O-M bonds is formed at an end of
the granular active material 110. For example, when carbon in the
granular active material 110 is represented by C, a functional
group is represented by OH or COOH, and the granular active
material 110 including the functional group is represented by C--OH
or C--COOH, condensation reaction is expressed by Formula 3 or
Formula 4.
Si(OEt).sub.4-x(OH).sub.x+C--OH.fwdarw.C--O--Si(OEt).sub.4-x(OH).sub.x-1-
+H.sub.2O (x is an integer of 4 or
less) (Formula 3)
Si(OEt).sub.4-x(OH).sub.x+C--COOH.fwdarw.C--CO--O--Si(OEt).sub.4-x(OH).s-
ub.x-1+H.sub.2O (x is an integer of
4 or less) (Formula 4)
[0078] The condensation reaction is determined by the kind of
hydrate of silicon ethoxide and the number of functional groups at
ends of the granular active material 110. Note that in the case
where an end of the granular active material 110 has a dangling
bond, carbon having a dangling bond bonds to the above oxide; thus,
a C--O-M bond is formed.
[0079] Through these steps, the net-like structure formed of C--O-M
bonds can be formed on part of the surface of the granular active
material 110.
[0080] After that, heat treatment is performed under atmospheric
pressure at Step S153, whereby the material for an electrode of a
power storage device can be produced. The temperature of the heat
treatment is higher than or equal to 300.degree. C. and lower than
or equal to 900.degree. C., preferably higher than or equal to
500.degree. C. and lower than or equal to 800.degree. C.
[0081] By the producing method using a sol-gel method shown in FIG.
4A, the electrode material in which the net-like structure formed
of C--O-M bonds is formed on part of the surface of the granular
active material can be produced.
[Method of Producing Electrode Material of Power Storage Device
Using Polysilazane Method]
[0082] At Step S160, a stabilizing agent is added to a solvent, and
the mixture is stirred to form a solution.
[0083] As the solvent, toluene can be used, for example.
[0084] As the stabilizing agent, ethyl acetoacetate can be used,
for example.
[0085] In the case where, for example, a silicon oxide film is
formed as the coating film 111, at Step S161, the granular active
material 110 and a polysilazane-containing solution which contains
perhydropolysilazane are added to the above solution, and the
mixture is stirred. Thus, the solution becomes a thick paste.
[0086] Next, at Step S162, the sample is kept in the air, and heat
treatment is performed at Step S163 so that inversion of
perhydropolysilazane is performed. Note that the heat treatment
temperature is higher than or equal to 30.degree. C. and lower than
or equal to 600.degree. C., preferably higher than or equal to
100.degree. C. and lower than or equal to 200.degree. C. The heat
treatment is not necessarily performed. For example, the inversion
of perhydropolysilazane can be performed by keeping the sample at
temperature higher than or equal to 15.degree. C. and lower than
30.degree. C. for a certain period of time. When
perhydropolysilazane is SiH.sub.2NH, the inversion reaction is
expressed by Formula 5.
SiH.sub.2NH+2H.sub.2O.fwdarw.SiO.sub.2+NH.sub.3+2H.sub.2 (Formula
5)
[0087] At this time, a compound including Si(OH) is generated as a
by-product. This compound reacts with a functional group at the end
of the granular active material 110, whereby the net-like structure
formed of C--O-M bonds is formed. For example, when carbon in the
granular active material 110 is represented by C, a functional
group is represented by OH or COOH, and the granular active
material 110 including the functional group is represented by C--OH
or C--COOH, the generation reaction is expressed by Formula 6 and
Formula 7.
Si(OH)+C--OH.fwdarw.C--O--Si--+H.sub.2O (Formula 6)
Si(OH)+C--COOH.fwdarw.C--CO--O--Si--+H.sub.2O (Formula 7)
[0088] Through these steps, the net-like structure formed of C--O-M
bonds can be formed on part of the surface of the granular active
material 110.
[0089] By the producing method using a polysilazane method shown in
FIG. 4B, the electrode material in which the net-like structure
formed of C--O-M bonds is formed on part of the surface of the
granular active material can be produced.
Embodiment 2
[0090] In this embodiment, a negative electrode of a power storage
device using the material for an electrode of a power storage
device described in Embodiment 1 and a method for forming the
negative electrode will be described with reference to FIGS. 5A to
5D.
[0091] As illustrated in FIG. 5A, a negative electrode 200 includes
a negative electrode current collector 201 and a negative electrode
active material layer 202 provided on one or both surfaces (on the
both surfaces in the drawing) of the negative electrode current
collector 201.
[0092] The negative electrode current collector 201 is formed using
a highly conductive material which is not alloyed with a carrier
ion such as lithium. For example, stainless steel, copper, nickel,
or titanium can be used. In addition, the negative electrode
current collector 201 can have a foil-like shape, a plate-like
shape (sheet-like shape), a net-like shape, a punching-metal shape,
an expanded-metal shape, or the like as appropriate. The negative
electrode current collector 201 preferably has a thickness of more
than or equal to 10 .mu.m and less than or equal to 30 .mu.m.
[0093] The negative electrode active material layer 202 is provided
on one or both surfaces of the negative electrode current collector
201. For the negative electrode active material layer 202, the
electrode material described in Embodiment 1 can be used.
[0094] In this embodiment, the negative electrode active material
layer 202 formed by mixing and baking the electrode material
described in Embodiment 1, a conductive additive, and a binder is
used.
[0095] The negative electrode active material layer 202 is
described with reference to FIG. 5B. FIG. 5B is a cross-sectional
view of part of the negative electrode active material layer 202.
The negative electrode active material layer 202 includes the
electrode material described in Embodiment 1, a conductive additive
204, and a binder (not illustrated).
[0096] The conductive additive 204 has a function of increasing the
conductivity between the granular negative electrode active
materials 203 or between the granular negative electrode active
material 203 and the negative electrode current collector 201, and
is preferably added to the negative electrode active material layer
202, for example, A material with a large specific surface is
desirably used as the conductive additive 204, and acetylene black
(AB) or the like can be used. Alternatively, a carbon material such
as a carbon nanotube, graphene, or fullerene can be used as the
conductive additive 204. Note that the case of using graphene is
described later as an example.
[0097] As the hinder, a material which at least binds the negative
electrode active material, the conductive additive, and the current
collector is used. Examples of the binder include resin materials
such as polyvinylidene fluoride (PVDF), a vinylidene
fluoride-hexafluoropropylene copolymer, a vinylidene
fluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymer
rubber, polytetrafluoroethylene, polypropylene, polyethylene, and
polyimide.
[0098] The negative electrode 200 is formed in the following
manner. First, the electrode material made by the method described
in Embodiment 1 is mixed to a solvent such as NMP
(N-methylpyrrolidone) in which a vinylidene fluoride based polymer
such as polyvinylidene fluoride is dissolved, whereby slurry is
formed.
[0099] Next, the slurry is applied on one or both surfaces of the
negative electrode current collector 201, and dried. In the case
where the application step is performed on both surfaces of the
negative electrode current collector 201, the negative electrode
active material layers 202 are formed on the surfaces at the same
time or one by one. Then, rolling with a roller press machine is
performed, whereby the negative electrode 200 is formed.
[0100] Next, an example of using graphene as the conductive
additive added to the negative electrode active material layer 202
is described with reference to FIGS. 5C and 5D.
[0101] FIG. 5C is a plan view of part of the negative electrode
active material layer 202 using graphene. The negative electrode
active material layer 202 includes the granular negative electrode
active materials 203 which correspond to the granular active
materials 110 described in Embodiment 1 and graphenes 205. The
graphene 205 covers a plurality of granular negative electrode
active materials 203 and at least partly surround the plurality of
granular negative electrode active materials 203. A binder which is
not illustrated may be added; however, in the case where the
graphenes 205 are contained so that they are bound to each other to
function well as a binder, the binder is not necessarily added. In
the plan view of the negative electrode active material layer 202,
different graphenes 205 cover the surfaces of the granular negative
electrode active materials 203. The granular negative electrode
active materials 203 may be partly exposed.
[0102] FIG. 5D is a cross-sectional view of part of the negative
electrode active material layer 202 in FIG. 5C. FIG. 5D illustrates
the granular negative electrode active materials 203 and the
graphenes 205. In the plan view of the negative electrode active
material layer 202, the graphene 205 covers a plurality of granular
negative electrode active materials 203. The graphene 205 has a
linear shape when observed in the cross-sectional view. The
plurality of granular negative electrode active materials 203 are
at least partly surrounded with one graphene 205 or a plurality of
graphenes 205 or sandwiched between the plurality of graphenes 205.
Note that the graphene 205 has a bag-like shape and the plurality
of granular negative electrode active materials 203 is surrounded
by the graphene 205 in some cases. In addition, the granular
negative electrode active materials 203 are partly not covered with
the graphene 205 and exposed in some cases.
[0103] The thickness of the negative electrode active material
layer 202 is preferably selected as appropriate in the range of 20
.mu.m to 150 .mu.m.
[0104] Note that the negative electrode active material layer 202
may be predoped with lithium. Predoping with lithium may be
performed in such a manner that a lithium layer is formed on a
surface of the negative electrode active material layer 202 by a
sputtering method. Alternatively, lithium foil is provided on the
surface of the negative electrode active material layer 202,
whereby the negative electrode active material layer 202 can be
predoped with lithium.
[0105] As an example of the granular negative electrode active
material 203, there is a material whose volume is expanded by
occlusion of carrier ions. Thus, the negative electrode active
material layer including such a material gets friable and is partly
broken by charge and discharge, which reduces the reliability
(e.g., cycle performance) of the power storage device.
[0106] However, even when the volume of the negative electrode
active material is expanded due to charge and discharge, the
graphene 205 partly covers the periphery of the granular negative
electrode active materials 203, which allows prevention of
dispersion of the negative electrode active materials and the
breakdown of the negative electrode active material layer. That is
to say, the graphene 205 has a function of maintaining the bond
between the positive electrode active materials even when the
volume of the positive electrode active material fluctuates by
charge and discharge. For this reason, a binder does not need to be
used in forming the negative electrode active material layer 202.
Accordingly, the proportion of the negative electrode active
material in the negative electrode active material layer 202 with
certain weight (certain volume) can be increased, leading to an
increase in charge and discharge capacity per unit weight (unit
volume) of the electrode.
[0107] The graphene 205 has conductivity and is in contact with a
plurality of granular negative electrode active materials 203;
thus, it also serves as a conductive additive. That is, a
conductive additive does not need to be used in forming the
negative electrode active material layer 202. Accordingly, the
proportion of the negative electrode active material in the
negative electrode active material layer 202 with certain weight
(certain volume) can be increased, leading to an increase in charge
and discharge capacity per unit weight (unit volume) of the
electrode.
[0108] Furthermore, the graphene 205 efficiently forms a sufficient
conductive path of electrons in the negative electrode active
material layer 202, which increases the conductivity of the
negative electrode 200.
[0109] Note that the graphene 205 also functions as a negative
electrode active material that can occlude and release carrier
ions, leading to an increase in charge capacity of the negative
electrode 200.
[0110] Next, a method for forming the negative electrode active
material layer 202 in FIGS. 5C and 5D is described.
[0111] First, the electrode material described in Embodiment 1 and
a dispersion liquid containing graphene oxide are mixed to form
slurry.
[0112] Next, the slurry is applied onto the negative electrode
current collector 201. Next, drying is performed in a vacuum for a
certain period of time to remove a solvent from the slurry applied
onto the negative electrode current collector 201. Then, rolling
with a roller press machine is performed.
[0113] Then, the graphene oxide is electrochemically reduced with
electric energy or thermally reduced by heating treatment to form
the graphene 205. Particularly in the case where electrochemical
reduction treatment is performed, a proportion of C(.pi.)--C(.pi.)
double bonds of graphene formed by the electrochemical reduction
treatment is higher than that of graphene formed by heating
treatment; therefore, the graphene 205 having high conductivity can
be formed. Through the above steps, the negative electrode active
material layer 202 used as a conductive additive can be formed on
one or both surfaces of the negative electrode current collector
201, and thus the negative electrode 200 can be formed.
Embodiment 3
[0114] In this embodiment, a structure of a lithium ion battery as
a power storage device and a method for manufacturing the lithium
ion battery are described.
(Positive Electrode)
[0115] First, a positive electrode and a method for forming the
positive electrode will be described.
[0116] FIG. 6A is a cross-sectional view of a positive electrode
250. In the positive electrode 250, a positive electrode active
material layer 252 is formed over a positive electrode current
collector 251.
[0117] For the positive electrode current collector 251, a highly
conductive material such as a metal typified by stainless steel,
gold, platinum, zinc, iron, aluminum, or titanium, or an alloy of
these metals can be used. Note that the positive electrode current
collector 251 can be formed using an aluminum alloy to which an
element which improves heat resistance, such as silicon, titanium,
neodymium, scandium, or molybdenum, is added. Further
alternatively, the positive electrode current collector 251 may be
formed using a metal element which forms silicide by reacting with
silicon. Examples of the metal element which forms silicide by
reacting with silicon include zirconium, titanium, hafnium,
vanadium, niobium, tantalum, chromium, molybdenum, tungsten,
cobalt, nickel, and the like. The positive electrode current
collector 251 can have a foil-like shape, a plate-like shape (a
sheet-like shape), a net-like shape, a punching-metal shape, an
expanded-metal shape, or the like as appropriate.
[0118] In addition to a positive electrode active material, a
conductive additive and a binder may be included in the positive
electrode active material layer 252.
[0119] As the positive electrode active material of the positive
electrode active material layer 252, a compound such as
LiFeO.sub.2, LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4,
V.sub.2O.sub.5, Cr.sub.2O.sub.5, or MnO.sub.2 can be used.
[0120] Alternatively, an olivine-type lithium-containing composite
phosphate (LiMPO.sub.4 (general formula) (M is one or more of
Fe(II), Mn(II), Co(l), and Ni(II))) can be used for the positive
electrode active material. Typical examples of the general formula
LiMPO.sub.4 include LiFePO.sub.4, LiNiPO.sub.4, LiCoPO.sub.4,
LiMnPO.sub.4, LiFe.sub.aNi.sub.bPO.sub.4,
LiFe.sub.aCo.sub.bPO.sub.4, LiFe.sub.aMn.sub.bPO.sub.4,
LiNi.sub.aCo.sub.bPO.sub.4, LiNi.sub.aMn.sub.bPO.sub.4
(a+b.ltoreq.1, 0<a<1, and 0<b<1),
LiFe.sub.cNi.sub.dCo.sub.ePO.sub.4,
LiFe.sub.cNi.sub.DMn.sub.ePO.sub.4,
LiNi.sub.cCo.sub.dMn.sub.ePO.sub.4 (c+d+e.ltoreq.1, 0<c<1,
0<d<1, and 0<e<1),
LiFe.sub.fNi.sub.gCo.sub.hMn.sub.iPO.sub.4 (f+g+h+i.ltoreq.1,
0<f<1, 0<g<1, 0<h<1, and 0<i<1), and the
like.
[0121] Alternatively, as the positive electrode active material, a
lithium-containing composite silicate represented by a general
formula Li(.sub.2-j)MSiO.sub.4 (M is one or more of Fe(II), Mn(II),
Co(II), and Ni(II); 0.ltoreq.j<2) can be used. Typical examples
of the general formula Li(.sub.2-j)MSiO.sub.4 include
Li(.sub.2-j)FeSiO.sub.4, Li(.sub.2-j)NiSiO.sub.4,
Li(.sub.2-j)CoSiO.sub.4, Li(.sub.2-j)MnSiO.sub.4,
Li(.sub.2-j)Fe.sub.kNi.sub.lSiO.sub.4,
Li(.sub.2-j)Fe.sub.kCo.sub.lSiO.sub.4,
Li(.sub.2-j)Fe.sub.kMn.sub.lSiO.sub.4,
Li(.sub.2-j)Ni.sub.kCo.sub.lSiO.sub.4,
Li(.sub.2-j)Ni.sub.kMn.sub.lSiO.sub.4 (k+l.ltoreq.1, 0<k<1,
and 0<l<1), Li(.sub.2-j)Fe.sub.mNi.sub.nCo.sub.qSiO.sub.4,
Li(.sub.2-j)Fe.sub.mNi.sub.nMn.sub.qSiO.sub.4,
Li(.sub.2-j)Ni.sub.mCo.sub.nMn.sub.qSiO.sub.4 (m+n+q.ltoreq.1,
0<m<1, 0<n<1, and 0<q<1),
Li(.sub.2-j)Fe.sub.rNi.sub.sCo.sub.tMn.sub.uSiO.sub.4
(r+s+t+u.ltoreq.1, 0<r<1, 0<s<1, 0<t<1, and
0<u<1), and the like.
[0122] In the case where carrier ions are alkaline-earth metal ions
or alkali metal ions other than lithium ions, the positive
electrode active material layer 252 may contain, instead of lithium
in the above lithium compound, lithium-containing composite
phosphate, and lithium-containing composite silicate, an alkali
metal (e.g., sodium or potassium) or an alkaline-earth metal (e.g.,
calcium, strontium, barium, beryllium, or magnesium).
[0123] The positive electrode active material layer 252 is not
necessarily formed in contact with the positive electrode current
collector 251. Between the positive electrode current collector 251
and the positive electrode active material layer 252, any of the
following functional layers may be formed using a conductive
material such as a metal: an adhesive layer for the purpose of
improving adhesiveness between the positive electrode current
collector 251 and the positive electrode active material layer 252,
a planarization layer for reducing unevenness of the surface of the
positive electrode current collector 251, a heat radiation layer
for radiating heat, and a stress relaxation layer for relieving
stress of the positive electrode current collector 251 or the
positive electrode active material layer 252.
[0124] FIG. 6B is a plan view of the positive electrode active
material layer 252. For the positive electrode active material
layer 252, granular positive electrode active materials 253 that
can occlude and release carrier ions are used. An example is shown
in which the positive electrode active material layer 252 includes
graphenes 254 covering a plurality of granular positive electrode
active materials 253 and at least partly surrounding the plurality
of granular positive electrode active materials 253. The different
graphenes 254 cover surfaces of the plurality of granular positive
electrode active materials 253. The granular positive electrode
active materials 253 may be partly exposed.
[0125] The particle diameter of the granular positive electrode
active material 253 is preferably greater than or equal to 20 nm
and less than or equal to 100 nm. Note that the particle diameter
of the granular positive electrode active material 253 is
preferably smaller because electrons transfer in the granular
positive electrode active material 253.
[0126] Although sufficient characteristics can be obtained even
when the surface of the granular positive electrode active material
253 is not covered with a graphite layer, it is preferable to use
the granular positive electrode active material 253 covered with a
graphite layer, in which case hopping of carrier ions occurs
between the granular positive electrode active materials 253, so
that current flows.
[0127] FIG. 6C is a cross-sectional view of part of the positive
electrode active material layer 252 in FIG. 6B. The positive
electrode active material layer 252 includes the granular positive
electrode active materials 253 and the graphenes 254 covering a
plurality of granular positive electrode active materials 253. The
graphene 254 has a linear shape when observed in the
cross-sectional view. The plurality of granular positive electrode
active materials 253 are at least partly surrounded with one
graphene 254 or a plurality of graphenes 254 or sandwiched between
the plurality of graphenes 254. Note that the graphene 254 has a
bag-like shape and the plurality of granular positive electrode
active materials 253 is surrounded by the graphene 254 in some
cases. In addition, the positive electrode active materials are
partly not covered with the graphene 254 and exposed in some
cases.
[0128] The desired thickness of the positive electrode active
material layer 252 is determined in the range of 20 .mu.m to 100
.mu.m. It is preferable to adjust the thickness of the positive
electrode active material layer 252 as appropriate so that cracks
and separation do not occur.
[0129] Note that the positive electrode active material layer 252
may contain a known conductive additive, for example, acetylene
black particles having a volume 0.1 to 10 times as large as that of
the graphene or carbon particles such as carbon nanofibers having a
one-dimensional expansion.
[0130] Depending on a material of the positive electrode active
material, the volume is expanded by occlusion of ions serving as
carriers. When such a material is used, the positive electrode
active material layer gets vulnerable and is partly collapsed by
charge and discharge, which results in lower reliability of a power
storage device. However, even when the volume of the positive
electrode active material expands due to charge and discharge, the
graphene partly covers the periphery of the positive electrode
active material, which allows prevention of dispersion of the
positive electrode active material and the breakage of the positive
electrode active material layer. That is to say, the graphene has a
function of maintaining the bond between the positive electrode
active materials even when the volume of the positive electrode
active materials fluctuates by charge and discharge.
[0131] The graphene 254 is in contact with the plurality of
positive electrode active materials and serves also as a conductive
additive. Further, the graphene 254 has a function of holding the
positive electrode active material capable of occluding and
releasing carrier ions. Thus, a binder does not have to be mixed
into the positive electrode active material layer. Accordingly, the
amount of the positive electrode active material in the positive
electrode active material layer can be increased, which allows an
increase in discharge capacity of non-aqueous secondary
batteries.
[0132] Next, description is given of a method for forming the
positive electrode active material layer 252.
[0133] First, slurry containing granular positive electrode active
materials and graphene oxide is formed. Next, the slurry is applied
onto the positive electrode current collector 251. Then, heating is
performed in a reduced atmosphere for reduction treatment so that
the positive electrode active materials are baked and oxygen
included in the graphene oxide is extracted to form graphene. Note
that oxygen in the graphene oxide is not entirely extracted and
partly remains in the graphene. Through the above process, the
positive electrode active material layer 252 can be formed over the
positive electrode current collector 251. Consequently, the
positive electrode active material layer 252 has higher
conductivity.
[0134] Graphene oxide contains oxygen and thus is negatively
charged in a polar solvent. As a result of being negatively
charged, graphene oxide is dispersed in the polar solvent.
Therefore, the positive electrode active materials contained in the
slurry are not easily aggregated, so that the particle diameter of
the positive electrode active material can be prevented from
increasing due to aggregation. Thus, the transfer of electrons in
the positive electrode active materials is facilitated, resulting
in an increase in conductivity of the positive electrode active
material layer.
[0135] Next, a structure and a method for manufacturing a lithium
secondary battery are described with reference to FIGS. 7A and 7B.
Here, a cross-sectional structure of the lithium ion secondary
battery is described below.
(Coin-Type Lithium Ion Battery)
[0136] FIG. 7A is an external view of a coin-type (single-layer
flat type) lithium ion battery, and FIG. 7B is a cross-sectional
view thereof.
[0137] In a coin-type lithium ion battery 300, a positive electrode
can 301 serving also as a positive electrode terminal and a
negative electrode can 302 serving also as a negative electrode
terminal are insulated and sealed with a gasket 303 formed of
polypropylene or the like. In a manner similar to that of the
above, a positive electrode 304 includes a positive electrode
current collector 305 and a positive electrode active material
layer 306 which is provided to be in contact with the positive
electrode current collector 305. On the other hand, a negative
electrode 307 includes a negative electrode current collector 308
and a negative electrode active material layer 309 which is
provided to be in contact with the negative electrode current
collector 308. A separator 310 and an electrolyte (not illustrated)
are included between the positive electrode active material layer
306 and the negative electrode active material layer 309.
[0138] As the negative electrode 307, the negative electrode
described in Embodiment 2 is used. As the positive electrode 304,
the positive electrode 250 described in this embodiment can be
used.
[0139] For the separator 310, an insulator such as cellulose
(paper), polypropylene with pores, or polyethylene with pores can
be used.
[0140] As an electrolyte of an electrolyte solution, a material
which contains carrier ions is used. Typical examples of the
electrolyte include lithium salts such as LiClO.sub.4, LiAsF.sub.6,
LiBF.sub.4, LiPF.sub.6, and Li(C.sub.2F.sub.5SO.sub.2).sub.2N.
[0141] In the case where carrier ions are alkali metal ions other
than lithium ions or alkaline-earth metal ions, the electrolyte may
contain, instead of lithium in the lithium salts, an alkali metal
(e.g., sodium or potassium), an alkaline-earth metal (e.g.,
calcium, strontium, barium, beryllium, or magnesium).
[0142] As a solvent of the electrolyte solution, a material in
which the carrier ions can transfer is used. As the solvent of the
electrolyte solution, an aprotic organic solvent is preferably
used. Typical examples of aprotic organic solvents include ethylene
carbonate (EC), propylene carbonate, dimethyl carbonate, diethyl
carbonate (DEC), .gamma.-butyrolactone, acetonitrile,
dimethoxyethane, and tetrahydrofuran, and one or more of these
materials can be used. With the use of a gelled high-molecular
material as the solvent of the electrolyte solution, safety against
liquid leakage and the like is improved. Furthermore, a lithium ion
battery can be thinner and more lightweight. Typical examples of
gelled high-molecular materials include a silicone gel, an acrylic
gel, an acrylonitrile gel, polyethylene oxide, polypropylene oxide,
and a fluorine-based polymer. Alternatively, the use of one or more
of ionic liquids (room temperature molten salts) which are less
likely to burn and volatilize as the solvent of the electrolyte
solution can prevent the lithium ion battery from exploding or
catching fire even when the secondary battery internally shorts out
or the internal temperature increases due to overcharging or the
like.
[0143] Instead of the electrolyte solution, a solid electrolyte
including a sulfide-based inorganic material, an oxide-based
inorganic material, or the like, or a solid electrolyte including a
polyethylene oxide (PEO)-based high-molecular material or the like
can be used. In the case of using the solid electrolyte, a
separator is not necessary. Further, the battery can be entirely
solidified; therefore, there is no possibility of liquid leakage
and thus the safety of the battery is dramatically increased.
[0144] For the positive electrode can 301 and the negative
electrode can 302, a corrosion-resistant metal such as nickel,
aluminum, or titanium, an alloy of such a metal, or an alloy of
such a metal and another metal (e.g., stainless steel or the like)
can be used. Alternatively, the positive electrode can 301 and the
negative electrode can 302 are preferably covered with nickel,
aluminum, or the like in order to prevent corrosion by the
electrolyte solution. The positive electrode can 301 and the
negative electrode can 302 are electrically connected to the
positive electrode 304 and the negative electrode 307,
respectively.
[0145] The negative electrode 307, the positive electrode 304, and
the separator 310 are immersed in the electrolyte solution. Then,
as illustrated in FIG. 7B, the positive electrode 304, the
separator 310, the negative electrode 307, and the negative
electrode can 302 are stacked in this order with the positive
electrode can 301 positioned at the bottom, and the positive
electrode can 301 and the negative electrode can 302 are subjected
to pressure bonding with the gasket 303 interposed therebetween. In
such a manner, the coin-type lithium ion battery 300 is
manufactured.
(Laminated Lithium Ion Battery)
[0146] Next, an example of a laminated lithium ion battery is
described with reference to FIG. 8.
[0147] In a laminated lithium ion battery 400 illustrated in FIG.
8, a positive electrode 403 including a positive electrode current
collector 401 and a positive electrode active material layer 402, a
separator 407, and a negative electrode 406 including a negative
electrode current collector 404 and a negative electrode active
material layer 405 are stacked and sealed in an exterior body 409,
and then an electrolyte solution 408 is injected into the exterior
body 409. Although FIG. 8 illustrates the laminated lithium ion
battery 400 with a structure in which one sheet-like positive
electrode 403 and one sheet-like negative electrode 406 are
stacked, to increase the capacity of the battery, the stack is
preferably wound or a plurality of positive electrodes and negative
electrodes are stacked and then laminated. Particularly in the case
of the laminated lithium ion battery, the battery has flexibility
and thus is suitable for applications which require
flexibility.
[0148] In the laminated lithium ion battery 400 illustrated in FIG.
8, the positive electrode current collector 401 and the negative
electrode current collector 404 serve as terminals for an
electrical contact with the outside. For this reason, the positive
electrode current collector 401 and the negative electrode current
collector 404 are arranged so that part of the positive electrode
current collector 401 and part of the negative electrode current
collector 404 are exposed outside the exterior body 409.
[0149] As the exterior body 409 in the laminated lithium ion
battery 400, for example, a laminate film having a three-layer
structure in which a highly flexible metal thin film of aluminum,
stainless steel, copper, nickel, or the like is provided over the
inner surface of a film formed of a material such as polyethylene,
polypropylene, polycarbonate, ionomer, or polyamide, and an
insulating synthetic resin film of a polyamide-based resin, a
polyester-based resin, or the like is provided as the outer surface
of the exterior body over the metal thin film can be used. With
such a three-layer structure, permeation of an electrolytic
solution and a gas can be blocked and an insulating property and
resistance to the electrolytic solution can be provided.
(Cylindrical Lithium Ion Battery)
[0150] Next, an example of a cylindrical lithium ion battery is
described with reference to FIGS. 9A and 9B. As illustrated in FIG.
9A, a cylindrical lithium ion battery 500 includes a positive
electrode cap (battery lid) 501 on its top surface and a battery
can (exterior can) 502 on its side surface and bottom surface. The
positive electrode cap 501 and the battery can 502 are insulated
from each other by a gasket 510 (insulating packing).
[0151] FIG. 9B is a diagram schematically illustrating a cross
section of the cylindrical lithium ion battery. In the battery can
502 with a hollow cylindrical shape, a battery element is provided
in which a strip-like positive electrode 504 and a strip-like
negative electrode 506 are wound with a separator 505 provided
therebetween. Although not illustrated, the battery element is
wound around a center pin as a center. One end of the battery can
502 is close and the other end thereof is open. For the battery can
502, a corrosion-resistant metal such as nickel, aluminum, or
titanium, an alloy of such a metal, or an alloy of such a metal and
another metal (e.g., stainless steel or the like) can be used.
Alternatively, the battery can 502 is preferably covered with
nickel, aluminum, or the like in order to prevent corrosion by the
electrolyte solution. Inside the battery can 502, the battery
element in which the positive electrode, the negative electrode,
and the separator are wound is interposed between a pair of
insulating plates 508 and 509 which face each other. Further, an
electrolyte solution (not illustrated) is injected inside the
battery can 502 in which the battery element is provided. An
electrolyte solution which is similar to that of the coin-type
lithium ion battery or the laminated lithium ion battery can be
used.
[0152] Although the positive electrode 504 and the negative
electrode 506 can be formed in a manner similar to that of the
positive electrode and the negative electrode of the coin-type
lithium ion battery, the difference lies in that, since the
positive electrode and the negative electrode of the cylindrical
lithium ion battery are wound, active materials are formed on both
sides of the current collectors. A positive electrode terminal
(positive electrode current collecting lead) 503 is connected to
the positive electrode 504, and a negative electrode terminal
(negative electrode current collecting lead) 507 is connected to
the negative electrode 506. A metal material such as aluminum can
be used for both the positive electrode terminal 503 and the
negative electrode terminal 507. The positive electrode terminal
503 is resistance-welded to a safety valve mechanism 512, and the
negative electrode terminal 507 is resistance-welded to the bottom
of the battery can 502. The safety valve mechanism 512 is
electrically connected to the positive electrode cap 501 through a
positive temperature coefficient (PTC) element 511. The safety
valve mechanism 512 cuts off electrical connection between the
positive electrode cap 501 and the positive electrode 504 when the
internal pressure of the battery increases and exceeds a
predetermined threshold value. The PTC element 511 is a heat
sensitive resistor whose resistance increases as temperature rises,
and controls the amount of current by increase in resistance to
prevent unusual heat generation. Barium titanate
(BaTiO.sub.3)-based semiconductor ceramic or the like can be used
for the PTC element.
[0153] Note that in this embodiment, the coin-type lithium ion
battery, the laminated lithium ion battery, and the cylindrical
lithium ion battery are given as examples of the lithium ion
battery; however, any of lithium ion batteries with various shapes,
such as a sealing-type lithium ion battery and a square-type
lithium ion battery, can be used. Further, a structure in which a
plurality of positive electrodes, a plurality of negative
electrodes, and a plurality of separators are stacked or wound may
be employed.
[0154] The electrode for a power storage device which is one
embodiment of the present invention is used as the negative
electrode in each of the lithium ion battery 300, the lithium ion
battery 400, and the lithium ion battery 500 described in this
embodiment. Thus, the lithium ion battery 300, the lithium ion
battery 400, and the lithium ion battery 500 can have favorable
cycle performance. For example, after 500 cycles of charge and
discharge, the capacity of the power storage device is preferably
higher than or equal to 60% of the initial capacity. In addition,
irreversible capacity generated in initial charge and discharge can
be reduced; moreover, a lithium ion battery with favorable high
temperature characteristics can be provided.
[0155] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 4
[0156] In this embodiment, a lithium ion capacitor is described as
a power storage device.
[0157] The lithium ion capacitor is a hybrid capacitor which
combines a positive electrode of an electrical double layer
capacitor (EDLC) and a negative electrode of a lithium ion battery
using a carbon material, and also an asymmetric capacitor in which
the principles of power storage are different between the positive
electrode and the negative electrode. The positive electrode forms
an electrical double layer and enables charge and discharge by a
physical action, whereas the negative electrode enables charge and
discharge by a chemical action of lithium. With the use of a
negative electrode in which lithium is occluded in advance as the
carbon material or the like that is a negative electrode active
material, the lithium ion capacitor can have energy density
dramatically higher than that of a conventional electrical double
layer capacitor including a negative electrode using active
carbon.
[0158] In the lithium ion capacitor, instead of the positive
electrode active material layer in the lithium ion battery
described in Embodiment 3, a material capable of reversibly having
at least one of lithium ions and anions is used. Examples of such a
material include active carbon, a conductive high molecule, and a
polyacene-based organic semiconductor (PAS).
[0159] The lithium ion capacitor has high efficiency of charge and
discharge, capability of rapidly performing charge and discharge,
and a long life even when it is repeatedly used.
[0160] As the negative electrode of such a lithium ion capacitor,
the negative electrode of a power storage device which is described
in Embodiment 2 is used. Thus, irreversible capacity generated in
initial charge and discharge is reduced, so that a power storage
device having improved cycle performance can be manufactured.
Furthermore, a power storage device having excellent high
temperature characteristics can be manufactured.
[0161] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 5
[0162] A power storage device of one embodiment of the present
invention can be used as a power supply of various electrical
appliances which are driven by electric power.
[0163] Specific examples of electrical appliances each using the
power storage device of one embodiment of the present invention are
as follows: display devices of televisions, monitors, and the like,
lighting devices, desktop personal computers and laptop personal
computers, word processors, image reproduction devices which
reproduce still images and moving images stored in recording media
such as digital versatile discs (DVDs), portable CD players,
portable radios, tape recorders, headphone stereos, stereos, table
clocks, wall clocks, cordless phone handsets, transceivers, mobile
phones, car phones, portable game machines, calculators, portable
information terminals, electronic notebooks, e-book readers,
electronic translators, audio input devices, video cameras, digital
still cameras, toys, electric shavers, high-frequency heating
appliances such as microwave ovens, electric rice cookers, electric
washing machines, electric vacuum cleaners, water heaters, electric
fans, hair dryers, air-conditioning systems such as air
conditioners, humidifiers, and dehumidifiers, dishwashers, dish
dryers, clothes dryers, futon dryers, electric refrigerators,
electric freezers, electric refrigerator-freezers, freezers for
preserving DNA, flashlights, electric power tools such as chain
saws, smoke detectors, and medical equipment such as dialyzers. The
examples also include industrial equipment such as guide lights,
traffic lights, belt conveyors, elevators, escalators, industrial
robots, power storage systems, and power storage devices for
leveling the amount of power supply and smart grid. In addition,
moving objects driven by an electric motor using power from a power
storage device are also included in the category of electrical
appliances. Examples of the moving objects are electric vehicles
(EV), hybrid electric vehicles (HEV) which include both an
internal-combustion engine and a motor, plug-in hybrid electric
vehicles (PHEV), tracked vehicles in which caterpillar tracks are
substituted for wheels of these vehicles, motorized bicycles
including motor-assisted bicycles, motorcycles, electric
wheelchairs, golf carts, boats, ships, submarines, helicopters,
aircrafts, rockets, artificial satellites, space probes, planetary
probes, and spacecrafts.
[0164] In the above electrical appliances, the power storage device
of one embodiment of the present invention can be used as a main
power source for supplying enough power for almost the whole power
consumption. Alternatively, in the above electrical appliances, the
power storage device of one embodiment of the present invention can
be used as an uninterruptible power source which can supply power
to the electrical appliances when the supply of power from the main
power source or a commercial power source is stopped. Still
alternatively, in the above electrical appliances, the power
storage device of one embodiment of the present invention can be
used as an auxiliary power source for supplying power to the
electrical appliances at the same time as the power supply from the
main power source or a commercial power source.
[0165] FIG. 10 illustrates specific structures of the electrical
appliances. In FIG. 10, a display device 600 is an example of an
electrical appliance using a power storage device 604 of one
embodiment of the present invention. Specifically, the display
device 600 corresponds to a display device for TV broadcast
reception and includes a housing 601, a display portion 602,
speaker portions 603, the power storage device 604, and the like.
The power storage device 604 of one embodiment of the present
invention is provided in the housing 601. The display device 600
can receive power from a commercial power source. Alternatively,
the display device 600 can use power stored in the power storage
device 604. Thus, the display device 600 can be operated with the
use of the power storage device 604 of one embodiment of the
present invention as an uninterruptible power source even when
power cannot be supplied from a commercial power source due to
power failure or the like.
[0166] A semiconductor display device such as a liquid crystal
display device, a light-emitting device in which a light-emitting
element such as an organic EL element is provided in each pixel, an
electrophoretic display device, a digital micromirror device (DMD),
a plasma display panel (PDP), or a field emission display (FED) can
be used for the display portion 602.
[0167] Note that the display device includes, in its category, all
of information display devices for personal computers,
advertisement displays, and the like in addition to TV broadcast
reception.
[0168] In FIG. 10, an installation lighting device 610 is an
example of an electrical appliance using a power storage device 613
of one embodiment of the present invention. Specifically, the
installation lighting device 610 includes a housing 611, a light
source 612, the power storage device 613, and the like. Although
FIG. 10 illustrates the case where the power storage device 613 is
provided in a ceiling 614 on which the housing 611 and the light
source 612 are installed, the power storage device 613 may be
provided in the housing 611. The installation lighting device 610
can receive power from a commercial power source. Alternatively,
the installation lighting device 610 can use power stored in the
power storage device 613. Thus, the installation lighting device
610 can be operated with the use of the power storage device 613 of
one embodiment of the present invention as an uninterruptible power
source even when power cannot be supplied from a commercial power
source due to power failure or the like.
[0169] Note that although the installation lighting device 610
provided in the ceiling 614 is illustrated in FIG. 10 as an
example, the power storage device of one embodiment of the present
invention can be used as an installation lighting device provided
in, for example, a wall 615, a floor 616, a window 617, or the like
other than the ceiling 614. Alternatively, the power storage device
can be used in a tabletop lighting device or the like.
[0170] As the light source 612, an artificial light source which
emits light artificially by using power can be used. Specifically,
an incandescent lamp, a discharge lamp such as a fluorescent lamp,
and a light-emitting element such as an LED or an organic EL
element are given as examples of the artificial light source.
[0171] In FIG. 10, an air conditioner including an indoor unit 620
and an outdoor unit 624 is an example of an electrical appliance
using a power storage device 623 of one embodiment of the present
invention. Specifically, the indoor unit 620 includes a housing
621, an air outlet 622, the power storage device 623, and the like.
Although FIG. 10 illustrates the case where the power storage
device 623 is provided in the indoor unit 620, the power storage
device 623 may be provided in the outdoor unit 624. Alternatively,
the power storage device 623 may be provided in both the indoor
unit 620 and the outdoor unit 624. The air conditioner can receive
power from a commercial power source. Alternatively, the air
conditioner can use power stored in the power storage device 623.
Particularly in the case where the power storage devices 623 are
provided in both the indoor unit 620 and the outdoor unit 624, the
air conditioner can be operated with the use of the power storage
device 623 of one embodiment of the present invention as an
uninterruptible power source even when power cannot be supplied
from a commercial power source due to power failure or the
like.
[0172] Note that although the split-type air conditioner including
the indoor unit and the outdoor unit is illustrated in FIG. 10 as
an example, the power storage device of one embodiment of the
present invention can be used in an air conditioner in which the
functions of an indoor unit and an outdoor unit are integrated in
one housing.
[0173] In FIG. 10, an electric refrigerator-freezer 630 is an
example of an electrical appliance using a power storage device 634
of one embodiment of the present invention. Specifically, the
electric refrigerator-freezer 630 includes a housing 631, a door
for a refrigerator 632, a door for a freezer 633, the power storage
device 634, and the like. The power storage device 634 is provided
inside the housing 631 in FIG. 10. The electric
refrigerator-freezer 630 can receive power from a commercial power
source. Alternatively, the electric refrigerator-freezer 630 can
use power stored in the power storage device 634. Thus, the
electric refrigerator-freezer 630 can be operated with the use of
the power storage device 634 of one embodiment of the present
invention as an uninterruptible power source even when power cannot
be supplied from a commercial power source due to power failure or
the like.
[0174] Note that among the electrical appliances described above, a
high-frequency heating apparatus such as a microwave oven and an
electrical appliance such as an electric rice cooker require high
power in a short time. The tripping of a circuit breaker of a
commercial power source in use of electrical appliances can be
prevented by using the power storage device of one embodiment of
the present invention as an auxiliary power source for supplying
power which cannot be supplied enough by a commercial power
source.
[0175] In addition, in a time period when electrical appliances are
not used, particularly when the proportion of the amount of power
which is actually used to the total amount of power which can be
supplied from a commercial power source (such a proportion referred
to as a usage rate of power) is low, power can be stored in the
power storage device, whereby the usage rate of power can be
reduced in a time period when the electrical appliances are used.
For example, in the case of the electric refrigerator-freezer 630,
power can be stored in the power storage device 634 in nighttime
when the temperature is low and the door for a refrigerator 632 and
the door for a freezer 633 are not often opened and closed. On the
other hand, in daytime when the temperature is high and the door
for a refrigerator 632 and the door for a freezer 633 are
frequently opened and closed, the power storage device 634 is used
as an auxiliary power source; thus, the usage rate of power in
daytime can be reduced.
[0176] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Embodiment 6
[0177] Next, a portable information terminal which is an example of
an electrical appliance is described with reference to FIGS. 11A to
11C.
[0178] FIGS. 11A and 11B illustrate a tablet terminal 650 that can
be folded. FIG. 11A illustrates the tablet terminal 650 in the
state of being unfolded. The tablet terminal 650 includes a housing
651, a display portion 652a, a display portion 652b, a switch 653
for switching display modes, a power switch 654, a switch 655 for
switching to power-saving-mode, and an operation switch 656.
[0179] Part of the display portion 652a can be a touch panel region
657a and data can be input when a displayed operation key 658 is
touched. Note that FIG. 1i A illustrates, as an example, that half
of the area of the display portion 652a has only a display function
and the other half of the area has a touch panel function. However,
the structure of the display portion 652a is not limited to this,
and all the area of the display portion 652a may have a touch panel
function. For example, all the area of the display portion 652a can
display keyboard buttons and serve as a touch panel while the
display portion 652b can be used as a display screen.
[0180] Like the display portion 652a, part of the display portion
652b can be a touch panel region 657b. When a finger, a stylus, or
the like touches the place where a button 659 for switching to
keyboard display is displayed in the touch panel, keyboard buttons
can be displayed on the display portion 652b.
[0181] Touch input can be performed on the touch panel regions 657a
and 657b at the same time.
[0182] The switch 653 for switching display modes can switch the
display between portrait mode, landscape mode, and the like, and
between monochrome display and color display, for example. With the
switch 655 for switching to power-saving mode, the luminance of
display can be optimized depending on the amount of external light
at the time when the tablet terminal is in use, which is detected
with an optical sensor incorporated in the tablet terminal. The
tablet terminal may include another detection device such as a
sensor for detecting orientation (e.g., a gyroscope or an
acceleration sensor) in addition to the optical sensor.
[0183] Although the display area of the display portion 652a is the
same as that of the display portion 652b in FIG. 11A, one
embodiment of the present invention is not particularly limited
thereto. The display area of the display portion 652a may be
different from that of the display portion 652b, and further, the
display quality of the display portion 652a may be different from
that of the display portion 652b. For example, one of them may be a
display panel that can display higher-definition images than the
other.
[0184] FIG. 11B illustrates the tablet terminal 650 in the state of
being closed. The tablet terminal 650 includes the housing 651, a
solar cell 660, a charge and discharge control circuit 670, a
battery 671, and a DCDC converter 672. Note that FIG. 11B
illustrates an example in which the charge and discharge control
circuit 670 includes the battery 671 and the DCDC converter 672,
and the battery 671 includes the power storage device described in
any of the above embodiments.
[0185] Since the tablet terminal 650 can be folded, the housing 651
can be closed when the tablet terminal 650 is not in use. Thus, the
display portions 652a and 652b can be protected, thereby providing
the tablet terminal 650 with excellent endurance and excellent
reliability for long-term use.
[0186] The tablet terminal illustrated in FIGS. 11A and 11B can
also have a function of displaying various kinds of data (e.g., a
still image, a moving image, and a text image), a function of
displaying a calendar, a date, the time, or the like on the display
portion, a touch-input function of operating or editing data
displayed on the display portion by touch input, a function of
controlling processing by various kinds of software (programs), and
the like.
[0187] The solar cell 660, which is attached on the surface of the
tablet terminal, supplies power to the touch panel, the display
portion, a video signal processor, and the like. Note that the
solar cell 660 is preferably provided on one or two surfaces of the
housing 651, in which case the battery 671 can be charged
efficiently. The use of the power storage device of one embodiment
of the present invention as the battery 671 has advantages such as
a reduction is size.
[0188] The structure and operation of the charge and discharge
control circuit 670 illustrated in FIG. 11B are described with
reference to a block diagram in FIG. 11C. The solar cell 660, the
battery 671, the DCDC converter 672, a converter 673, switches SW1
to SW3, and the display portion 652 are illustrated in FIG. 11C,
and the battery 671, the DCDC converter 672, the converter 673, and
the switches SW1 to SW3 correspond to the charge and discharge
control circuit 670 illustrated in FIG. 11B.
[0189] First, an example of the operation in the case where power
is generated by the solar cell 660 using external light is
described. The voltage of power generated by the solar cell 660 is
raised or lowered by the DCDC converter 672 so that the power has a
voltage for charging the battery 671. Then, when the power from the
solar cell 660 is used for the operation of the display portion
652, the switch SW1 is turned on and the voltage of the power is
raised or lowered by the converter 673 so as to be a voltage needed
for the display portion 652. In addition, when display on the
display portion 652 is not performed, the switch SW1 may be turned
off and the switch SW2 may be turned on so that the battery 671 is
charged.
[0190] Here, the solar cell 660 is described as an example of a
power generation means; however, there is no particular limitation
on the power generation means, and the battery 671 may be charged
with another power generation means such as a piezoelectric element
or a thermoelectric conversion element (Peltier element). For
example, the battery 671 may be charged with a non-contact power
transmission module that transmits and receives power wirelessly
(without contact) to charge the battery or with a combination of
other charging means.
[0191] It is needless to say that one embodiment of the present
invention is not limited to the electrical appliance illustrated in
FIGS. 11A to 11C as long as the electrical appliance is equipped
with the power storage device described in any of the above
embodiments.
Embodiment 7
[0192] Further, an example of the moving object which is an example
of the electrical appliance is described with reference to FIGS.
12A and 12B.
[0193] Any of the power storage device described in any of the
above embodiments can be used as a control battery. The control
battery can be externally charged by electric power supply using a
plug-in technique or contactless power feeding. Note that in the
case where the moving object is an electric railway vehicle, the
electric railway vehicle can be charged by electric power supply
from an overhead cable or a conductor rail.
[0194] FIGS. 12A and 12B illustrate an example of an electric
vehicle. An electric vehicle 680 is equipped with a battery 681.
The output of the power of the battery 681 is adjusted by a control
circuit 682 and the power is supplied to a driving device 683. The
control circuit 682 is controlled by a processing unit 684
including a ROM, a RAM, a CPU, or the like which is not
illustrated.
[0195] The driving device 683 includes a DC motor or an AC motor
either alone or in combination with an internal-combustion engine.
The processing unit 684 outputs a control signal to the control
circuit 682 based on input data such as data on operation (e.g.,
acceleration, deceleration, or stop) by a driver of the electric
vehicle 680 or data on driving the electric vehicle 680 (e.g., data
on an upgrade or a downgrade, or data on a load on a driving
wheel). The control circuit 682 adjusts the electric energy
supplied from the battery 681 in response to the control signal of
the processing unit 684 to control the output of the driving device
683. In the case where the AC motor is mounted, although not
illustrated, an inverter which converts direct current into
alternate current is also incorporated.
[0196] The battery 681 can be charged by external electric power
supply using a plug-in technique. For example, the battery 681 is
charged through a power plug from a commercial power source. The
battery 681 can be charged by converting external power into DC
constant voltage having a predetermined voltage level through a
converter such as an ACDC converter. When the power storage device
of one embodiment of the present invention is provided as the
battery 681, capacity of the battery 681 can be increased and
improved convenience can be realized. When the battery 681 itself
can be made compact and lightweight with improved characteristics
of the battery 681, the vehicle can be made lightweight, leading to
an increase in fuel efficiency.
[0197] Note that it is needless to say that one embodiment of the
present invention is not limited to the electrical appliance
described above as long as the power storage device of one
embodiment of the present invention is included.
[0198] This embodiment can be implemented in combination with any
of the other embodiments as appropriate.
Example
[0199] In this example, a negative electrode of a power storage
device, which includes a coating film formed of a silicon oxide,
and a power storage device using the negative electrode, which are
actually manufactured, will be described.
Reference Example
[0200] As a reference example, description will be made on an
example of an electrode in which a silicon oxide film is formed
over a layer serving as an active material.
[0201] In the reference example, a 100-.mu.m-thick titanium sheet
TR270C produced by JX Nippon Mining & Metals Corporation was
used as a current collector, and a 200-nm-thick silicon film was
formed over the current collector by a thermal CVD method.
Moreover, SiO.sub.2 powder was pelletized and deposited by electron
beam heating, whereby a 100-nm-thick silicon oxide film was formed
over the silicon film. Thus, an electrode (also referred to as an
electrode A) was formed. An electrode (also referred to as an
electrode B) was formed as a comparative example by forming only a
200-nm-thick silicon film over a current collector formed with the
same material as the current collector in the electrode A.
[0202] A three-electrode cell (also referred to as a cell A) in
which the electrode A is used as a working electrode and a
three-electrode cell (also referred to as a cell B) in which the
electrode B is used as a working electrode were fabricated. In this
case, lithium was used for a reference electrode and a counter
electrode in a three-electrode electrochemical measurement cell. In
addition, an electrolyte solution was formed in such a way that
lithium hexafluorophosphate (LiPF.sub.6) was dissolved at a
concentration of 1 mol/L in a solution in which ethylene carbonate
(EC) and diethyl carbonate (DEC) were mixed at a volume ratio of
1:1.
[0203] The cell A and the cell B are subjected to cyclic
voltammetry (CV) measurement. The potential range in the CV
measurement was 0 V to 2 V (vs. Li/Li), and scanning was performed
only in the negative direction. The sweep rate in an electric field
was set to 0.1 mV/sec. FIG. 13 is the cyclic voltammogram showing
the CV measurement results.
[0204] FIG. 13 indicates that even in the case where the silicon
film is covered with the silicon oxide film, lithium ions are
inserted into the silicon film serving as an active material. The
silicon oxide film has a function of allowing passage of lithium
ions, and lithium ions react with the silicon film.
(Example of Power Storage Device)
[0205] As an example, a negative electrode of a power storage
device and a power storage device using the negative electrode,
which are actually manufactured, will be described.
[0206] In this example, graphite particles provided with a silicon
oxide film were formed by a sol-gel method. For the graphite
particles, graphite produced by JFE Chemical Corporation was used.
First, silicon ethoxide, ethyl acetoacetate, and toluene were mixed
and stirred to form a Si(OEt).sub.4 toluene solution, as described
in Embodiment 1. At this time, the amount of silicon ethoxide was
determined so that the proportion of silicon oxide formed later in
graphite was 1 wt % (weight percent). The compounding ratio of this
solution was as follows: Si(OEt).sub.4 was 3.14.times.10.sup.-4
mol; ethyl acetoacetate, 6.28.times.10.sup.-4 mol; and toluene, 2
ml. Next, graphite was added to the Si(OEt).sub.4 toluene solution
and the mixture was stirred in a dry room. Then, the solution was
held at 70.degree. C. in a humid environment for 3 hours so that
Si(OEt).sub.4 in the Si(OEt).sub.4 toluene solution to which
graphite was added was hydrolyzed and condensed. In other words,
Si(OEt).sub.4 in the solution was made to react with water in the
air to gradually cause hydrolysis reaction, and Si(OEt).sub.4 was
condensed by dehydration reaction which sequentially occurred. In
such a manner, silicon which is a gel was attached onto a surface
of graphite particles to form a net-like structure formed of
C--O--Si bonds. Then, baking was performed in a nitrogen atmosphere
at 500.degree. C. for 3 hours, thereby forming an electrode
material containing the graphite particles covered with a coating
film formed of a silicon oxide. In addition, slurry formed by
mixing the electrode material, acetylene black, and PVDF was
applied onto a current collector and dried; thus, an electrode
(also referred to as Electrode 1) was formed. At this time, the
weight ratio of PVDF to graphite was 10 wt % (weight percent).
[0207] FIG. 14 is an observation image of Electrode 1 taken with a
scanning electron microscope (SEM). FIG. 14 shows that a plurality
of particles 2010 is formed. The plurality of particles has an
average diameter of approximately 9 .mu.m.
[0208] In addition, observation with a scanning transmission
electron microscope (STEM) and energy dispersive X-ray spectroscopy
(EDX) were performed on Electrode 1. FIGS. 15A and 15B show the
results of the observation and analysis. The results of EDX in
FIGS. 15A and 15B are obtained by line scanning.
[0209] In the STEM image in FIG. 15A, a relatively dark gray
portion corresponds to the particle 2010. The plurality of
particles 2010 can be observed.
[0210] A relatively light gray region 2011 exists between the
plurality of particles 2010. From the result of EDX, silicon is
detected in the line A-B across part of the particle 2010 region
and part of the light gray region 2011. This indicates that the
light gray region 2011 corresponds to the silicon oxide film. On
the other hand, from the STEM image in FIG. 15B and the result of
EDX in the line C-D, the silicon oxide film is not observed.
Therefore, it is found that the silicon oxide film is not formed on
the entire surface of the particles 2010, but formed on part of the
surface of the particles 2010.
(CV Measurement)
[0211] Next, whether the silicon oxide film of Electrode 1 has an
effect of suppressing decomposition of the electrolyte solution or
not was examined by CV measurement.
[0212] For the CV measurement, a three-electrode cell was used, an
electrode X was used as a working electrode, lithium was used for a
reference electrode and a counter electrode, and a solution
obtained by dissolving 1 mol/L of lithium hexafluorophosphate
(LiPF.sub.6) in a mixed solution of ethylene carbonate (EC) and
diethyl carbonate (DEC) (volume ratio 1:1) was used as an
electrolyte solution. The measurement was performed at a scan rate
(the sweep rate in an electric field) of 4 .mu.V/sec in the
potential range of 0.01 V to 1 V (vs. Li/Li+).
[0213] FIGS. 16A and 16B show the results of CV measurement of one
cycle. FIG. 16A shows the results of measurement in the scan range
of 0.01 V to 1 V. FIG. 16B is a graph focused on potentials around
0.4 V to 1V.
[0214] In FIG. 16B, a peak 2002 appears in the range of 0.7 V to 1
V. This indicates decomposition of the electrolyte solution.
[0215] For the comparison, Electrode 2 in which a silicon oxide
film is not provided on a surface of graphite particles which are
the same as the above-described graphite particles was made, and CV
measurement was performed for two cycles under the same conditions.
FIGS. 17A and 17B show the results obtained by comparing a cell
using Electrode 1 with a cell using Electrode 2. FIG. 17A is a
cyclic voltammogram, and FIG. 17B is a graph showing the capacity
of decomposition of the electrolyte solution, which is calculated
on the basis of the results in FIG. 17A.
[0216] As shown in FIGS. 17A and 17B, the peak 2002 that appears in
the range of 0.7 V to 1 V in the cell using Electrode 2 is higher
than that in the cell using Electrode 1. Therefore, decomposition
of the electrolyte solution can be suppressed by providing a
silicon oxide film.
(Cycle Performance Evaluation)
[0217] A negative electrode X and a negative electrode Y were
formed. In the negative electrode X, graphite particles provided
with a silicon oxide film, which is formed by the above-mentioned
sol-gel method, are used as negative electrode active materials. In
the negative electrode Y, graphite particles provided with a
silicon oxide film, which is formed by the above-mentioned
polysilazane method, are used as negative electrode active
materials. A cell using LiFePO.sub.4 as a positive electrode and
the negative electrode X and a cell using LiFePO.sub.4 as a
positive electrode and the negative electrode Y were formed, and
the cycle performance of the cells were compared with each
other.
[0218] The negative electrode X using a sol-gel method was made in
a manner similar to that of Electrode 1.
[0219] In the formation of the negative electrode Y using a
polysilazane method, 5 g of graphite produced by JFE Chemical
Corporation and 2.5 ml of toluene were mixed in a dry room; 1.3 mg
of a xylene solution containing 20 wt % perhydropolysilazane was
added thereto; and the mixture was further mixed in the dry room.
The mixture was kept in the air for 30 minutes, subjected to heat
treatment with a hot plate in the air at 150.degree. C. for one
hour, and dried with a glass tube oven at 170.degree. C. for 0
hours. Thus, an electrode material including graphite particles
provided with a silicon oxide film was formed. In addition, slurry
formed by mixing the electrode material, acetylene black, and PVDF
was applied onto a current collector of copper with a thickness of
18 .mu.m and dried. At this time, the weight ratio of PVDF to
graphite was 10 wt %/o (weight percent).
[0220] Note that the negative electrode X and the negative
electrode Y were made such that the weight ratio of the silicon
oxide film to the graphite particles was 1 wt % (weight
percent).
[0221] The performance was measured using coin cells. An
electrolyte solution formed in such a manner that lithium
hexafluorophosphate (LiPF.sub.6) was dissolved at a concentration
of 1 mol/L in a solution in which ethylene carbonate (EC) and
diethyl carbonate (DEC) were mixed at a volume ratio of 1:1 was
used. As the separator, polypropylene (PP) was used. Charge and
discharge were performed at a rate of 1 C (it takes 1 hour for
charging), voltages ranging from 2 V to 4 V, and an environment
temperature of 60.degree. C. Under such conditions, the measurement
was performed.
[0222] The cycle performance evaluation was performed on a
secondary battery using the negative electrode X, a secondary
battery using the negative electrode Y, and a secondary battery
using a negative electrode Z. The negative electrode Z was made for
comparison, in which graphite particles which are not provided with
the coating film are used as negative electrode active
materials.
[0223] FIG. 18 shows the results of the cycle performance
evaluation. The horizontal axis represents the number of cycles
(times) and the vertical axis represents capacity retention (%) of
the secondary batteries. The number of measured samples of each of
the secondary battery using the negative electrode X, the secondary
battery using the negative electrode Y, and the secondary battery
using the negative electrode Z was two (n=2).
[0224] FIG. 18 shows that as the number of cycles increases, the
discharge capacities of the secondary batteries using the negative
electrode X and secondary batteries using the negative electrode Y
are less likely to decrease than those of the secondary batteries
using the negative electrode Z at 60.degree. C. For example, after
500 cycles of charge and discharge, the capacity of the secondary
battery using the negative electrode X is higher than or equal to
60% of the initial capacity. Thus, decomposition reaction of the
electrolyte solution, which speeds up at high temperature, is
suppressed and a decrease in capacity in charge and discharge at
high temperature is suppressed, so that the operating temperature
range of a power storage device can be extended.
[0225] The decrease in discharge capacity of the secondary battery
using the negative electrode X is smaller than the decrease in
discharge capacity of the secondary battery using the negative
electrode Y. A net-like structure is less likely formed on the
negative electrode active material produced by a polysilazane
method as compared with on the negative electrode active material
produced by a sol-gel method. This is because Si(OH) is needed for
forming the net-like structure, and the amount of Si(OH) generated
by the method of producing the electrode material using a
polysilazane method is smaller than the amount of Si(OH) generated
by the method of producing the electrode material using a sol-gel
method. Therefore, formation of the net-like structure leads to
improvement in cycle performance and reliability of a power storage
device and.
[0226] This application is based on Japanese Patent Application
serial No. 2012-224581 fled with Japan Patent Office on Oct. 9,
2012, the entire contents of which are hereby incorporated by
reference.
* * * * *